The present invention relates to a technique for generating multi-channel music signals from two-channel music signals.
A multi-channel surround technique is available in which a plurality of speakers is arranged so as to surround the listener and sound is output from the respective speakers so as to envelop the listener, thereby enhancing presence. As the arrangement positions of the respective speakers in the multi-channel surround technique, for example, five speakers including a center channel speaker C, a left front speaker L, a right front speaker R, a left surround speaker SL and a right surround speaker SR are arranged at the corresponding positions. The left front speaker L and the right front speaker R are arranged on the left side and the right side of the front respectively as viewed from the listener and are used for sound image localization on the front left side, the direct front and the front right side. The left surround speaker SL and the right surround speaker SR are arranged on the left side (or the left rear side) and the right side (or the right rear side) of the listener respectively and are used for sound image localization on the sides and the rear sides of the listener and for reproduction of non-localized sound. The center channel speaker C is arranged on the direct front of the listener and used to reproduce the sound localized on the front of the listener, for example, words of a movie. Although this kind of multi-channel surround technique has been used frequently, for example, for acoustic reproduction in movie theaters and the like, the technique is also used, for example, for acoustic reproduction in the so-called home theaters and video games.
Acoustic signals to be reproduced are required to conform to the multi-channel surround technique to perform acoustic reproduction being rich in presence in home theaters and video games. For this reason, even if, for example, a movie on a DVD (digital versatile disc) having been recorded by the related-art stereo system is reproduced by devices conforming to the multi-channel surround technique, the listener cannot enjoy sound with presence. Hence, for the purpose of solving this kind of problem, various techniques (hereafter referred to as up-mixing techniques) have been proposed in which the stereo audio signals of the left and right two-channels are processed beforehand so that the individual channel signals can be extracted and audio signals to be supplied to the respective speakers of a multi-channel surround system are generated. As the up-mixing techniques, Dolby Pro Logic (registered trade mark) and the technique disclosed in U.S. Pat. No. 7,003,467 are available, for example.
In the matrix signal processing of Dolby Pro Logic (registered trade mark), for example, the respective left and right two-channel audio signals (left channel audio signal and the right channel audio signal) are added (or subtracted) while being subjected to gain adjustment so as to generate an audio signal to be supplied to each speaker of the multi-channel surround system. For example, the audio signal to be supplied to the surround speaker is generated as the signal (L−R) obtained by subtracting the right-channel audio signal from the left channel audio signal. In this case, the audio signal to be supplied to the surround speaker is extracted as the opposite-phase component in the audio signals of the left and right channels.
Such an up-mixing technique as Dolby Pro Logic (registered trade mark) described above is suited for processing in which a plurality of signals including words and BGMs having been separated distinctly as in movie contents is down-mixed to left and right two-channel signals. On the other hand, in the case that channel extension is performed by carrying out the above-mentioned matrix signal processing for acoustic signals not subjected to down-mixing, such as ordinary music signals, a delay (effect) for intentionally shifting sound emission timing, for example, is erroneously determined as a signal of an opposite-phase component (surround channel), whereby there is a risk that unintentional reproduction processing may be carried out. Hence, a technique for generating multi-channel audio signals from two-channel acoustic signals, different from the above-mentioned up-mixing technique for use in movies and the like, is demanded.